This laboratory has been studying the complex of enzymes synthesizing deoxyribonucleotides which are induced after bacteriophage T4 infection. We have shown that this complex is integrated with DNA synthesis, that certain of these enzymes have a second direct role in DNA replication and that ribonucleotides are channeled to the DNA growing point. The system intrinsically synthesizes dTMP and HMdCMP derivatives in the ratio of 2:1, i.e., as in T4 DNA, whether or not DNA is being formed. DNA has an intimate, apparently structural role in the regulation. We are pursuing these studies in animal cultures. BIBLIOGRAPHIC REFERENCES: Greenberg, G.R., Chakrabarti, P., and Khorana, H.G. (1976) Incorporation of Fatty Acids Containing Photosensitive Groups Into Phospholipids of Escherichia coli, Proc. Natl. Acad. Sci. USA 73, 86. Chiu, C.-S., Tomich, P.K., and Greenberg, G. R. (1976) Simultaneous Initiation of Synthesis of Bacteriophage T4 DNA and of Deoxyribonucleotides, Proc. Natl. Acad. Sci. USA 73 (March 1976).